The proposed research involves the continuation of ultrastructural and biochemical investigations into the development of microvilli. The specific aims of this project include: (1) examining biochemical and ultrastructural aspects of actin filament bundling during formation of intestinal epithelia cell microvilli, epdidymal principal cell sterecilla, and inner ear hair stereocilia; and (2) analyzing ultrastructurally and biochemically the association of filament bundles with the plasma membrane. These studies should allow for a better understanding of a phenomenon common to many kinds of cellular movements; that is, the extension or retraction of a cellular process. The proposed studies should also provide new information that will complement existing literature leading to a general model for microvillus growth. The research proposed within is comparative in nature and therefore should be generally applicable to many systems. Biochemical work currently in progress and already completed in the applicant's laboratory has provided some exciting new results concerning the attachment protein(s) linking microvillar core filament bundles to the plasma membrane. Research proposed in this grant should allow the isolation and purification of this protein(s) along with noting its appearance and localization during the development and growth of microvilli. Ultrastructural work nearing completion on the development of mouse intestinal epithelial cell microvilli and mouse epididymal principal cell stereocilia has provided new evidence that: (1) comments on the modifies the two existing models for microvillus growth, and (2) suggest that there are at least two processes involved in microvillus growth (i.e., 1) assembly of a bundle of actin filaments attached to the plasma membrane, and 2) association, through lateral arms, of the core filament bundle with the plasma membrane). Only through more detailed electron microscopic and biochemical analysis will a fuller understanding of microvillus development be acquired.